I'll rate every student of class A based on how likely they are to be the traitor.

Anything with a 5/10 or higher has a decent chance imo.

Let's go.

Kaminari: 9/10

There is a big theory in the internet that is based in the fact he was weirdly absent during the camp invasion arc. His character is also a good fit, because he lacks a real arc, and is one-dimensional enough to be expanded from (his whole thing is being stupid, so turns out he's smart all along). His quirk/design are good fits for a villain too, and he could replace Magne.

Hakagure: 9/10

The other one with a good popular theory, but instead of the camp arc, she disappeared in the USJ arc (she was shown with Todoroki in one scene, and in the next one he is beating villains alone). She is the most one-dimensional character in the class, and no feats outside of an anime-only fight against Snipe, so this would be a good way to develop her.

Ashido: 7/10

Ok this is a crazy theory but don't hit the "back" button yet. Ashido is weird because she has a lot of screentime but is probably Bottom 3 in feats. She has never done anything. Her biggest moment is in Kirishima's flashback. Ok, so what if all this inspiring and optimistic personality is a facade? And then, it would take Kirishima's arc in a whole new direction, with him trying ro redeem her. Plus acid is a dope villain power.

Aoyama: 6/10

I have no idea lol. The author just used him as a red herring for it, but could he be a double bait? He is so alien that I don't know. His personality isn't really villain-like though.

Jiro: 5/10

She lacks enough of a character arc to make it possible, but she seems to come from a functional family so that doesn't make much sense. Plus she seems to be written as an underdog of sorts, not really the evil-all-along type.

Shoji: 5/10

There is nothing really against it but... I don't see the point. His thing is being the scary-looking guy who is actually pretty nice so making him not-nice-all-along is... dumb? I don't see any emotion in it.

Sato: 5/10

Another character with a depressing lack in feats and arc that leaves room for it, but his design just screams "decent guy". I don't see it.

Sero: 5/10

He is bland enough to allow for an expansion. But I kind of like how he is written as this guy with a silly power who is somehow super reliable, and who is dumb not Kaminari-dumb. His design doesn't feel like villain either.

Ojiro: 5/10

Another minor character which allows for it, but his whole thing is being the guy with an incredible lame power who somehow gets to be a super-hero. Making him a villain... kind of defeats the point? Plus he seems to have a honorable-like personality.

Koda: 4/10

Next minor character in the list. His character arc void allows it but... can you seriously look at Koda's face and imagine him as a bad guy? I can't.

Momo: 4/10

Like Ochako below, her traitor status is betrayed by her monologues showing how she is worried about not being a fit. It could be stretched to make her a double agent all along, but it's somewhat trollish.

Tsuyu: 3/10

Despite the screentime she has enough gaps in arc and backstory to make it narratively possible, but everything in her design screams against it, from her rescue/support quirk to her personality. And the crying scene contradicts it. Next.

Mineta: 3/10

It's technically possible but his design is too lame for a villain. He is the joke character. Plus he is only barely tolerable because he's a good guy, do we want Mineta to be made more creepy? I don't.

Ochako: 2/10

There is precedent for a character like her to turn evil (Kaede from Assassination Classroom). The problem is her huge number of monologues. Her being evil contradicts all of them. No.

Kirishima: 2/10

He has an arc now, and the arc doesn't have room for it.

Tokoyami: 2/10

If he was the traitor why did the villains try to recruit him? Unless this traitor thing is a AfO-only thing. This is a bit crazy tho. Also his quirk fits more with the hero who has trouble controlling it than with an all-out evil guy.

Todoroki: 2/10

He has an arc too. Maybe he could be a villain to spite his father? I have a hard time seeing how that can be written in a way that's not really dumb though. This isn't Naruto.

Bakugou: 1/10

Hori had a chance at making him evil when he was kidnapped but passed on it. He could still turn evil later but he's not the traitor.

Despite the screentime she has enough gaps in arc and backstory to make it narratively possible, but everything in her design screams against it, from her rescue/support quirk to her personality. And the crying scene contradicts it. Next.

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what if she was faking ?
Her secret plan with the villains to acquire Bakugou failed so she concocted this random crying scene to try and play innocent

Despite the screentime she has enough gaps in arc and backstory to make it narratively possible, but everything in her design screams against it, from her rescue/support quirk to her personality. And the crying scene contradicts it. Next.

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The crying doesn't really have to contradict Asui possibly being the traitor. She's shown to have a couple of little siblings so its a theory that she could serve as the traitor not because she wants to but to protect her family. This would make the Asui we see all the time the real one and a potential traitor asui being a role she forces herself to play.

Kaminari 9/10: Kaminari would be a perfect person to be the traitor, considering his popularity and relationship with the other characters in Class 1-A. Jiro, Kirishima, Bakugou, and Mineta would all get moments of character development if they were to find out that Kaminari was the traitor. While some might argue that he isn't the traitor due to the USJ incident, I feel like the traitor was someone who was recruited after the USJ incident, seeing as the villains didn't know anything about the students (look at Tsuyu being brought to the water area), they could've recruited him for his great quirk afterwards. Plus, he was one of the ones safe and away from the battle at the camp.

Hagakure 9/10: An over jovial schoolgirl with a stereotypical personality that could very well be faked. We've never heard her inner thoughts or seen her expressions. We've never seen who she is looking at in scenes. Her quirk leaves a lot to be questioned, and makes for being a perfect traitor. What if this whole time she could control her invisibility, but Aizawa could never turn it off since he has to see his opponent to disable their quirk?

On top of that, the fact that she was with Todoroki at USJ, but did nothing to help him, and didn't go with him to help stop Shigaraki... kinda creepy, isn't it?

On top of that, we saw inside her room. A typical girls room. Nothing out of the ordinary... suspicious. On top of that, there was attention brought to her drawers. Sure, as just a gag for Mineta... but what if they brought relevance to it in the future? What if she was hiding something?

Mina 7/10: A popular character for her looks and personality, but what do we know of her? She didn't try fighting Kurogiri back at USJ, and was in the building during the camp attack. We know she has a cheery personality, but just like Hagakure, that could be all an act. Plus, her acidic quirk would make for a great villainous quirk, and would add another female char to the roster of villains. While it's not as likely as the others, it would be kinda cool, and would affect quite a few characters in the class. She's sort of the one people don't expect, which would make it rather shocking, all the while making a good amount of sense.

Mineta 6/10: This would make for a cool villain flip since everybody hates Mineta for his lame personality. But what if it was faked? What if secretly he was using this personality to masquerade his true character? He was absent during the fighting in the camp arc when the villains attacked, so it could line up. Make himself a scumbag that nobody wants to pay attention to, then be able to scheme more easily.

There are some doubts in my head with this one though, and that's... well, look at Bakugou and Endeavor. They're 2 of the most problematic characters in the series. Hori has showcased that he likes taking flawed characters and making them learn, and I feel like Mineta could go through some good growth that has more potential than him ending up as a villain. So I kinda hope it's not him, and that he sticks with UA.

Tsuyu 5/10: Tsuyu as a villain would be... interesting. It would explain why she's so eerily calm all the time, perhaps it's because she's not who we think she is. But she's competed in so many different events, that I really have to wonder... Specifically, she was attacked by Toga. Perhaps that was her playing it as an act with her, but Toga did physically touch her, and Tsuyu did help Shoji and the others chase after Compress, sooo... having a hard time believing it. If Hori makes a convincing way to explain all of it, I can dig it, but... meh.

As for the rest, they're either characters that haven't had enough screentime or personality to make them interesting as traitors (ojiro and sato), or they've competed in far too many events saving people against the villains (shoji on both occassions), or have had inner thoughts about wanting to be a hero (todoroki, iida, kouda, etc).

That wouldn't be too farfetch'd. He fakes his quirks' limitations and can hold a copy of someone's quirk for at least 30 mins to an hour. Or maybe he could copy, abliet a weaker version, of someone's quirk permenately.

Not to mention he's in Yuuhei and can see/talk/reach all the upcoming heroes and pros as much as he wants to. And the Pros would have no real way of knowing his real strength or identity.

I personally think it is GravityGirl who is the traitor because that would be the easiest thing to do. Slip out at night and come back in the morning. Nobody would even be able to see you come/go unless you wanted them to.

I also personally think it could also be the animal talking guy. I mean if he can talk to animals far and wide how the fuck would anyone be able to stop him or detect him doing that? From the outside looking in it could seem he is just being bubbly cute with squirrels but on the real he is giving them top secret shit to take back to the LOV. I think that would be a hilarious fan drawing if someone drew him telling a squirel, "Deku's arms will break if he uses them again. Tell Shira!" while the girls are just looking at him going, "D'awwwww! Isnt' he so cute!"
There is also the possibility that the Principal of the school is an animal and can be controlled by the animal talker so the real culprit and traitor is the fucking principal himself hahaha. He just doesn't know it because he gets controlled which is a mini brain-wash.

Another possibilty that is probably not likely but I still had the idea anyway. Luke/Leia/Anakin or Rey/Ben have this 'force' connection where they can tap into the other person's thoughts for a brief second or some have visions and shit. I could see AFO/OFA having this connection even though it may be one sided and he needs concentration with AM being totally oblivious to the fact he can see inside his soul/person. Sort of like Harry/Voldy.
All this time AM wonders why he was always one step behind LOL. It could be that he was being spied on the entire time without knowing it. Then this would give way of AM having to die for the sake of peace sort of like how Goku had to die in DBZ (or so he thought.)

I personally think it is GravityGirl who is the traitor because that would be the easiest thing to do. Slip out at night and come back in the morning. Nobody would even be able to see you come/go unless you wanted them to.

I also personally think it could also be the animal talking guy. I mean if he can talk to animals far and wide how the fuck would anyone be able to stop him or detect him doing that? From the outside looking in it could seem he is just being bubbly cute with squirrels but on the real he is giving them top secret shit to take back to the LOV. I think that would be a hilarious fan drawing if someone drew him telling a squirel, "Deku's arms will break if he uses them again. Tell Shira!" while the girls are just looking at him going, "D'awwwww! Isnt' he so cute!"
There is also the possibility that the Principal of the school is an animal and can be controlled by the animal talker so the real culprit and traitor is the fucking principal himself hahaha. He just doesn't know it because he gets controlled which is a mini brain-wash.

Another possibilty that is probably not likely but I still had the idea anyway. Luke/Leia/Anakin or Rey/Ben have this 'force' connection where they can tap into the other person's thoughts for a brief second or some have visions and shit. I could see AFO/OFA having this connection even though it may be one sided and he needs concentration with AM being totally oblivious to the fact he can see inside his soul/person. Sort of like Harry/Voldy.
All this time AM wonders why he was always one step behind LOL. It could be that he was being spied on the entire time without knowing it. Then this would give way of AM having to die for the sake of peace sort of like how Goku had to die in DBZ (or so he thought.)

Anyone whose inner monologue we've seen/heard is probably safe from suspicion (going under the idea that the person is a willing traitor and it's not mind control, because then there's no point to speculating, it could literally be anyone in that case, even Midoriya and All Might). Like people have pointed out with Froppy, physical actions and emotions can be faked. There's no point in faking inside your own head.

Personally, I think it's Hagakure, and I kind of oddly hope she is too. I really like all the 1-A girls but I still think they need to be more involved in the bigger/heavier parts of the story, and having one of them be the traitor would certainly qualify.

I think we can rule out some characters as being the traitor, like Bakugou for one.

I mean if he was the traitor, why the hell would he let himself get kidnapped by the VA when he could just simply leave at anytime and go join them?

Also can safely assume to rule out Momo, Deku, Kirishima, Shoto and Iida too. I mean if any one of those guys where the traitor, why they would risk their necks trying to rescue Kacchan if that was the case?

What im getting at is... Does AFO or the VA really want to take down the school that badly? Why?

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Well, UA is known as the best hero course in the world, so it's where most of the future's best heroes would be coming from. Just like how taking down All Might would shatter society's sense of safety and stability, so too would happen taking down UA. If the villains are looking to destroy hero society/the society that promotes false heroes, naturally they're going to strike at the biggest symbols of said society that they can find.

Well, UA is known as the best hero course in the world, so it's where most of the future's best heroes would be coming from. Just like how taking down All Might would shatter society's sense of safety and stability, so too would happen taking down UA. If the villains are looking to destroy hero society/the society that promotes false heroes, naturally they're going to strike at the biggest symbols of said society that they can find.

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Basically this, also, from a more meta-level, it's basically free characterization and development because you pretty much have to explore all the details about how it all went down, as well as audience engagement. Ongoing background plot threads like double agents in fiction are the dirty little secrets for retaining an audience, keeps people guessing an analysing each and every possibility whenever a character is on screen. Not to mention, whoever the traitor is is only someone capable of leaking info, which while not exactly the biggest thing ever, the implications of their actions have been felt greatly(It's basically their fault All Might retired) so they pose a big, BIG threat to the school security as well as the safety and well being of anyone affiliated with the students, including their parents(There's a bit of a popular sad end where Shigaraki or some other villains kidnaps or kills Deku's mom).

Basically this, also, from a more meta-level, it's basically free characterization and development because you pretty much have to explore all the details about how it all went down, as well as audience engagement. Ongoing background plot threads like double agents in fiction are the dirty little secrets for retaining an audience, keeps people guessing an analysing each and every possibility whenever a character is on screen. Not to mention, whoever the traitor is is only someone capable of leaking info, which while not exactly the biggest thing ever, the implications of their actions have been felt greatly(It's basically their fault All Might retired) so they pose a big, BIG threat to the school security as well as the safety and well being of anyone affiliated with the students, including their parents(There's a bit of a popular sad end where Shigaraki or some other villains kidnaps or kills Deku's mom).

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i can imagine deku's mom getting kidnapped ergo that is why we will see deku's dad
he comes in breathing fucking fire hotter than endeavor and is actually a top secret #0 hero that is stronger than all might
he was so powerful he had to go underground

i can imagine deku's mom getting kidnapped ergo that is why we will see deku's dad
he comes in breathing fucking fire hotter than endeavor and is actually a top secret #0 hero that is stronger than all might
he was so powerful he had to go underground

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Damn it, now I want there to be a rank 0 hero. Why the hell is Rank 0 such a hype title?

i can imagine deku's mom getting kidnapped ergo that is why we will see deku's dad
he comes in breathing fucking fire hotter than endeavor and is actually a top secret #0 hero that is stronger than all might
he was so powerful he had to go underground

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Midoriya's dad finally being brought into the story because his mother is kidnapped? Love it.

Having him be a secret amazing top ten or beyond tier hero? I kind of hate the idea, I'm not going to lie.

Kind of the same reason I don't want anyone in Midoriya's family to have a blood connection with All For One or One For All. I think it hurts the protagonist's story if everything that's happened around them feels like destiny, like they didn't have any real choice in anything or that their choices didn't matter because they'd eventually be led that direction anyway. "Of course Midoriya can become a great hero, because his father IS a great hero."

Midoriya's dad finally being brought into the story because his mother is kidnapped? Love it.

Having him be a secret amazing top ten or beyond tier hero? I kind of hate the idea, I'm not going to lie.

Kind of the same reason I don't want anyone in Midoriya's family to have a blood connection with All For One or One For All. I think it hurts the protagonist's story if everything that's happened around them feels like destiny, like they didn't have any real choice in anything or that their choices didn't matter because they'd eventually be led that direction anyway. "Of course Midoriya can become a great hero, because his father IS a great hero."

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I do really hope Deku's dad is just a guy. Not an asshole. Not a deadbeat. Not this older Deku. Just a guy. And focus his reaction to Deku evolving into this hero in training and getting involved with all this as well as the circumstances alongside why he's back.

What im getting at is... Does AFO or the VA really want to take down the school that badly? Why?

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The traitor was probably planted in UA as Shiggy's initial goal was to kill All Might who was a teacher there- hence to set up the USJ Nomu attack. Even after All Might ceased to be a factor, its still useful to keep him around, especially since its quite likely All Might's successor is a student from UA.

i can imagine deku's mom getting kidnapped ergo that is why we will see deku's dad
he comes in breathing fucking fire hotter than endeavor and is actually a top secret #0 hero that is stronger than all might
he was so powerful he had to go underground